The present invention relates to radar declutter systems and especially to a method of and an apparatus for generating and receiving frequency-diversity, wideband, pulse-compression waveforms that minimize the effect of clutter returns in radar displays.
Clutter returns tend to mask the display of targets on a radar scope so that the target cannot be seen. Clutter may be considered to consist of an erratically varying signal that is specified in terms of a complex reflection density which has both an amplitude and a phase component. At a specified range delay, the transmitted radar pulse samples the clutter reflection density amplitude and phase components over a surface area that is equivalent to the pulse width. The sampled clutter amplitude is related to the pulse width, antenna beamwidth, depression angle and range from the radar, and is the product of the amplitudes of the clutter patch and the radar pulse. The sampled phase response is the sum of the clutter patch response and the pulse range phase response. The resultant clutter signal at the radar receiver consists of the phasor sum of the returns from various clutter patches illuminated by the radar pulse. The clutter may be assumed to consist of a discrete collection of point targets called clutter patches, arbitrarily dispersed along a reflecting surface with arbitrary radar reflecting properties, with some mutual but negligible coupling between the clutter patches.
Clutter may be minimized by the decorrelation of clutter signals returned from succeeding radar transmitted pulses. Clutter may be decorrelated on the basis of frequency diversity of successive radar pulses and also spatially decorrelated by increasing the radar pulse emission bandwidth. Thus, if a transmitted, wideband, expanded pulse waveform, which spatially decorrelates clutter, is frequency-hopped from pulse to pulse by the proper frequency amount and the returned radar signals are first compressed and delayed by one pulse interval and then correlated with the next received signal, clutter patches will be decorrelated. The frequency separation between two succeeding pulses, for frequency decorrelation, must be equal to at least the reciprocal of the compressed pulse width or equal to the emission bandwidth of the transmitted waveform. This technique is used by the present invention and will operate with both coherent and noncoherent frequency-diversity waveforms.